Chromatograph having a gas storage system

ABSTRACT

A chromatograph includes an inlet for receiving a sample and a pressurized hydrogen gas flow and in response providing a sample/fluid mixture; a separation column located in a temperature-controlled zone for receiving the sample/fluid mixture and for providing a column effluent stream; a detector for receiving the effluent stream and for providing a detector output stream; and a gas storage system for receiving the detector output stream (and optionally a split flow and a septum purge flow in the instance of a split/splitless inlet) and for storing the received gas stream for subsequent reuse. In the preferred embodiment of the gas storage system, a plurality of metal hydride storage (MHS) systems are used.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is related to commonly-assigned U.S. patentapplication Ser. No. ______, filed on even date herewith, in the name ofWilliam H. Wilson, and entitled “CHROMATOGRAPH HAVING A GAS RECYCLINGSYSTEM”.

FIELD OF THE INVENTION

[0002] The present invention relates to methods and apparatus for use inanalytical instrumentation for the detection of an analyte in a carrierfluid, and in particular to analytical instrumentation having aclosed-loop system for storage and reuse of hydrogen carrier gas.

BACKGROUND OF THE INVENTION

[0003] A simplified schematic view of a conventional chromatograph 100is shown in FIG. 1. The illustrated chromatograph 100 is representativeof a Hewlett-Packard 6890 Gas Chromatograph. Analytical instruments suchas the gas chromatograph 100 are known for use in determining thechemical composition of a sample which is typically injected at an inlet112 into a carrier gas stream provided by a carrier gas source 111through a manifold 113. A fluid mixture of the sample and the carriergas is directed through a separation column 114 located within an oven116 and exposed to a controlled temperature environment provided by aheater 118. The separation column 114 includes a stationary phasecoating on the interior of the column. The interaction of theconstituent compounds in the sample with the stationary phase causediffering chemical compounds in the sample to travel through theseparation column at different rates and to leave the separation columnat different times. The presence of compounds in the column effluent gasis sensed by a detector 124. A detector output signal is provided to acontroller 126 and a computer 122 on signal lines 128, 130. The compoundof interest typically called an analyte.

[0004] A significant shortcoming in the conventional gas chromatographis due to the loss of one or more gas streams that are typically ventedto the atmosphere from the inlet 112 or the detector 124. The majorityof the composition of such streams is carrier gas; for example, if theinlet 112 is constructed as a split/splitless inlet, much of the carriergas employed by the chromatograph 100 is vented away from the inlet 112.Accordingly, a column with a 1 ml/min flow rate and a split ratio of100:1 will vent 100 times the amount of gas actually required to carry asample through the column 114 for an analysis. Six liters of carrier gasat inlet pressure are typically lost to the surrounding environmentduring one hour of analysis. However, if the carrier gas were to beconserved, such a volume of gas could easily supply a column flow formany more hours of continuous operation.

[0005] The high rate of consumption of carrier gas observed in theconventional apparatus is one of the major factors that have inhibitedthe development of portable instrumentation, and has also limited thedeployment of most bench top (i.e., non-portable) chromatographs inunderdeveloped areas of the world where cylinders of carrier gas are inshort supply.

[0006] There thus exists a need for analytical instrumentation thatemploys a carrier gas storage system wherein, among other factors, theflow of the carrier fluid is conserved and reused to an extentsatisfactory for most analytical applications.

SUMMARY OF THE INVENTION

[0007] The advantages of the invention are achieved in a preferredembodiment of an analytical instrument, preferably provided in the formof a chromatograph, wherein a closed loop carrier gas storage systemreceives the gas streams that would be otherwise be vented to theatmosphere in a conventional apparatus, filters the received gas streamsin order to remove the residual impurities, and stores the filtered gasstream in a gas storage system, such that the stored gas may thereafterbe reused.

[0008] The preferred embodiment includes an inlet for receiving a sampleand a pressurized stream of carrier gas supplied from a carrier gasreservoir, and in response, providing a sample/fluid mixture and (insome embodiments hat operate a split/splitless inlet) an inlet outputstream in the form of a combination of a spit flow and a purge flaw; aseparation column located in a temperature-controlled compartment forreceiving the sample/fluid mixture and for providing a column effluentstream; a detector for receiving the column effluent stream andproviding in response a detector output signal and a detector outputstream; and a gas storage system for filtering and storing the receivedgas streams for subsequent reuse. The detector generates an outputsignal, whereby one or more characteristics of the effluent stream thatare related to the analyte of interest may be represented by the outputsignal.

[0009] Certain embodiments may further include a control systemincluding a computer for sensing the volumetric flow rate of the fluidmixture entering the column and for generating a respective flow ratesignal and for sensing the column input pressure and generating arespective input pressure signal, and an electronic pneumatic controllerincluding means for receiving the flow rate signal and input pressuresignal, for controlling the valve so as to control the input pressureand the volumetric flow rate of the carrier fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a simplified block diagram of an analytical instrumentconstructed as a chromatograph in accordance with the prior art.

[0011]FIG. 2 is a simplified schematic view of a preferred embodiment ofan analytical instrument constructed according to the present invention.

[0012]FIG. 3 is a simplified schematic view of first preferredembodiment of a gas storage system operable in the instrument of FIG. 2.

[0013]FIG. 4 is a simplified schematic view of second preferredembodiment of a gas storage system operable in the instrument of FIG. 2.

[0014] FIGS. 5A-5C are simplify schematic representations of the valvesequence operable in the second preferred embodiment of the gas storagesystem of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0015] The present invention will find useful application in a varietyof analytical systems that operate with use of hydrogen gas as a carriergas for detection of an analyte present in one or more carrier gasstreams. Gases are the preferred fluids according to the practice of thepresent invention, and hydrogen gas is the preferred carrier gas, andtherefore the following description of the invention will include adescription of the arrangement, construction, and operation of a gaschromatographic analytical system (hereinafter, a chromatograph).

[0016] For the purposes of the following description, the terms “fluid”and “pneumatic” will be considered to pertain to all types of fluids.“Fluid-handling function” refers to at least one of the followingfunctions with respect to one or more fluid streams: initiation;distribution; and redirection; termination; control of temperature,pressure or flow rate; and sensing of temperature, pressure, or flowrate. “Fluid-handling functional device” refers to a device thatprovides one or more fluid-handling functions with respect to one ormore fluid streams. “Electronic pneumatic control” and “EPC” refers toprogrammed electronic control of fluids and fluid handling functions,among which are included the control of volumetric flow rate andpressure of a fluid stream in a chromatograph, as for example inaccordance with the invention disclosed by U.S. Pat. No. 4,994,096, andU.S. Pat. No. 5,108,466 in the names of Klein, et al., the disclosuresof which are incorporated herein by reference, and to subsequentadvances known in the art for programmed electronic pneumatic control ofpressure, temperature, and/or flow rate of fluids in a chromatograph.

[0017] In a significant departure from the prior art, the presentinvention will be understood to overcome a major problem ofchromatographic systems that employ a conventional carrier gas source,and also will be understood to provide improved detection of a widerange of analytes present in a fluid stream.

[0018] In the embodiments illustrated in the Figures and describedbelow, like nomenclature and numerical identifiers refer to identical orequivalent structures; a single line indicates an electronic signal linecapable of transmitting an electronic signal; and double parallel linesindicate a pneumatic flow path capable of bearing a fluid stream.

[0019] A first preferred embodiment of a analytical instrument is shownin FIG. 2 and is generally designated as a chromatograph 200. Theillustrated chromatograph 200 is designed to operate using hydrogen gasas its carrier fluid. Accordingly, a hydrogen gas storage system 150 isinitially charged with hydrogen gas. The manifold 113 is especiallyconfigured to include selectable pneumatic pathways for receiving thecombine hydrogen gas flows that would otherwise be vented from thechromatograph 200 (such as from the detector 124 and/or the inlet 112).For example, in an embodiment wherein the inlet 112 is optionallyprovided in the form of a split/splitless inlet, the carrier gas flowfrom the storage system 150 is divided by the manifold 113 into threestreams. A first stream is directed to a septum purge line, a secondstream is directed to a split line, and a third stream is directed tothe column 114. A chemical trap (not shown) may be employed in themanifold 113 on the split line to prevent most of the sample materialfrom passing through the split line, such that the compounds capable oftraversing the chemical trap are restricted to volatile compounds. Theoutput of the column 114, septum purge line, and the split line may thenbe combined in manifold 113 and directed under the control of thecontroller 126 as an “incoming” gas stream to the gas storage system150. As will now be described, the gas storage system 150 then filtersthe incoming gas stream and stores the hydrogen gas component of suchincoming gas stream for subsequent reuse as the carrier gas needed bythe chromatograph 100.

[0020] As illustrated in FIG. 3, a first preferred embodiment of a gasstorage system 300 may be constructed to include first and second valvesV1, V2; flow restrictors R, a filter F, and first and second metalhydride storage (MHS) systems S1, S2 each having heaters H. The fitter Fmay be provided in the form of a chemical filter, an adsorbent trap, ora noble metal membrane that passes only hydrogen. In certainapplications, the gas storage system 150 may also incorporate a pumpingsystem constructed in the form of an electro-chemical pump (not shown)operable to boost the pressure of the incoming gas flow so as to bettercharge the metal hydride storage systems S1, S2.

[0021] A storage cycle may be understood as follows. The system S1 isinitially fully charged with hydrogen provided by an external source(not shown), and the system S2 is initially evacuated. The system S1 isheated to approximately 60 degrees Centigrade while the system S2remains at ambient temperature. The heated environment in system S1generates approximately 100 PSI of hydrogen pressure in system S1 andallows valve V2 to be actuated to provide hydrogen gas to the storagesystem output, such that the inlet 112 receives the hydrogen gas at aselectable hydrogen gas pressure. Meanwhile, an “incoming” gas stream isdeveloped from the gas streams from the inlet 112 and/or detector 124that would ordinarily be vented to the atmosphere. The incoming gasstream is first passed through filter F and the resulting filtered gasstream is directed to the evacuated system S2 through a restrictor R.The system S2 then begins to charge with hydrogen while the system S1continues to be depleted due to the demands of the chromatograph 200 forhydrogen gas. When the system S1 is nearly depleted, the system S2 isheated to 60 degrees Centigrade and the vales V1, V2 are rusted toprovide hydrogen gas to the system output from the system S2, and toredirect the incoming flow to the system S1. As the system S1 drops intemperature, it begins to absorb hydrogen from the incoming flow thusreceived, thereby storing the hydrogen for subsequent re-use. Thisstorage cycle may be repeated so as to provide a sufficient anduninterrupted supply of carrier gas for repeated use of thechromatograph 200.

[0022] As illustrated in FIGS. 4-5, a second preferred embodiment of agas storage system 400 may be constructed to include certain componentsof like nomenclature and accordingly of identical construction to thosealready described with reference to FIG. 3, but with the addition of athird metal hydride storage (MHS) system S3 and third and fourth valvesV3, V4. As shown in FIG. 5A, the system S1 is initially charged withhydrogen and systems S2 and S3 are evacuated. Valve V1 directs theincoming flow to valve V2 so as to bypass system S and thereafter intothe system S2. Meanwhile, system S1 is heated to generate an elevatedhydrogen gas pressure therein. Valve V3 directs a hydrogen gas flow fromsystem S1 to valve V4 which then directs the hydrogen gas flow to thestorage system output. System S2 continues to collect hydrogen gas whilesystem S3 is idle. As shown in FIG. 5B, when system S1 is sufficientlydepleted of hydrogen gas, valve V1 is actuated so as to direct theincoming flow received at the storage system input to system S3. ValveV3 is actuated to isolate systems S1 and S3. Valve V4 is actuated todirect a flow of hydrogen gas from the newly charged system S2 to thestorage system output. As shown in FIG. 5C, valves V1 and V2 may beactuated to direct incoming flow to the system S1. Valves V2 and V4 areactuated to isolate system S2 and valve V3 is set to allow hydrogen gasto flow from system S3 through valves V3 and V4 to the storage systemoutput.

[0023] In the above described embodiments, the hydrogen gas iscirculated in a closed loop storage system. Accordingly, thechromatograph 200 can operate for an indefinite period. For example, theonly path by which hydrogen gas would be lost in chromatograph 200 wouldbe by leaks present in the pneumatic lines, fittings, connectors, andother hydrogen bearing portions of the chromatograph 200. The propensityfor such leaks may be reduced by techniques and practices known to thoseskilled in the art.

[0024] Storage of hydrogen gas is an advantage for the construction ofportable or hand-held chromatographs, remotely situated “online”chromatographs for environmental monitoring or process control, andbench top chromatographs for use in underdeveloped countries where asupply of conventional gas cylinders is unreliable or simply notfeasible.

[0025] Although hydrogen permits the fastest chromatography, its use hasheretofore been avoided because of the potential exit hazard.Accordingly, because the carrier gas is conserved, the illustratedchromatograph 200 may be constructed for safe operation with hydrogencarrier gas. That is, the hydrogen gas may be maintained in a closedloop such that much of the hazard presented by the use of hydrogen gasis contained. This would permit the chromatograph 200 to be placed in anadverse environment without compromising most safety requirements.

[0026] Furthermore, the storage of the hydrogen gas in a closed loopsystem minimizes the chances for contamination of Wee hydrogen gasstream. The analytical performance of the illustrated chromatograph 200is accordingly enhanced.

[0027] While the invention has been described and illustrated withreference to specific embodiments, those skilled in the art willrecognize that modification and variations may be made without departingfrom the principles of the invention as described herein above and setforth in the following claims.

What is claimed is:
 1. A chromatograph for analysis of an analyte,comprising: an inlet for receiving a stream of hydrogen gas and a samplecontaining the analyte, and in response, providing a sample/fluidmixture; a separation column connected to the inlet for receiving thesample/fluid mixture and for providing a column effluent stream; adetector for receiving the effluent stream and for providing a detectoroutput stream, whereby the detector provides a detector signalrepresentative of a characteristic of the analyte; and a gas storagesystem for receiving a incoming gas stream provided in the form of atleast one of the detector output stream and a portion of the hydrogengas stream not present in the sample/fluid mixture, filtering theincoming gas stream to provide a filtered stream, and storing thefiltered stream so as to retain a conserved quantity of hydrogen gassuitable for reuse in the chromatograph as the storage system outputflow.
 2. The chromatograph of claim 1 , further comprising: a sensor forsensing the volumetric flow rate of the fluid mixture entering thecolumn and for generating a respective flow rate signal; and a pneumaticcontroller means for receiving the flow rate signal and for controllingin response the volumetric flow rate of the carrier fluid.
 3. Thechromatograph of claim 1 , further comprising an electronic pressurecontroller and wherein the flow of the hydrogen gas fluid stream issubject to control by the electronic pressure controller, wherebyunwanted flow rate variation in the hydrogen gas fluid stream subject todetection is reduced.
 4. The chromatograph of claim 1 , wherein the gasstorage system further comprises a filter for receiving the incoming gasstream and for passing only hydrogen gas to a hydrogen storage medium.5. The chromatograph of claim 4 , wherein the hydrogen storage mediumfurther comprises a metal hydride storage (MHS) system.
 6. Thechromatograph of claim 4 , wherein the filter further comprises, inseries, a packed trap for retaining non-volatile compounds and a noblemetal device permeable only by hydrogen.